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          KVO
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        <p>KVO用于逻辑隔离对象之间的监听，支持一对一和一对多的属性监听。这里的一对一和一对多是针对监听的属性的，即既可以监听单个属性，也可以监听集合属性。</p>
<p>在OC中，所有NSObject子类的所有属性（包括计算属性）都支持KVO；而在Swift中，只有在<code>@objc dynamic</code>修饰的属性（包括计算属性）才支持KVO，即使用<code>@objc dynamic</code>修饰的属性与OC行为一致。</p>
<p>KVC和KVO都属于键值编程，而且底层实现机制都是<strong>isa-swizzing</strong>。</p>
<p>KVO和NSNotificationCenter都是iOS观察着模式的一种实现。KVO对被监听对象无侵入性，即无需修改代码即可支持监听。</p>
<h2 id="使用">使用</h2>
<p>注册监听：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">[bankInstance addObserver:personInstance forKeyPath:@&quot;accountBalance&quot; options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];</span><br></pre></td></tr></table></figure>
<p><code>bankInstance</code>是变更发出的对象。<code>personInstance</code>是响应变更的对象。<code>@"accountBalance"</code>是<code>bankInstance</code>的key path。</p>
<p>选项是多选的，影响change字典内容和生成通知的方式。</p>
<ul>
<li>new：change字典包含新值</li>
<li>old：change字典包含旧值</li>
<li>initial：注册监听时即发送变更通知</li>
<li>prior：在变更通知前发送一次变更前的通知，即每个变更两次通知，一个willChange，一个didChange</li>
</ul>
<p>change字典的键：</p>
<ul>
<li>kind：改变类型
<ul>
<li>setting：对象或集合赋值替换</li>
<li>insertion：对象插入到集合属性中</li>
<li>removal：对象从集合中移除</li>
<li>replacement：对象从集合中替换</li>
</ul></li>
<li>new：新值</li>
<li>old：旧值</li>
<li>indexes：插入/移除/替换对象的索引</li>
<li>prior：标识该通知是willChange的，否则是didChange的</li>
</ul>
<p>new、old可以时单个对象，也可以时对象集合，表示集合时表示移除/替换的对象。</p>
<p>移除监听可在这两个时机完成：</p>
<ul>
<li>不需要监听时。</li>
<li>被观察对象、观察者对象销毁时。被观察对象与观察者对象常常具有相同的生命周期。</li>
</ul>
<p>使用注意：</p>
<ul>
<li><p>移除监听的调用角色与注册监听保持一致。</p></li>
<li><p>多次注册监听导致多次响应。didChnage变更通知的顺序与注册顺序相反，即以先进后出的顺序调用。</p></li>
<li><p>注册监听与移除监听必须保持配对，多了少了都会导致异常崩溃。但iOS 11以上不会崩溃。</p></li>
</ul>
<h3 id="手动处理变更通知">手动处理变更通知</h3>
<ol type="1">
<li>在被观察对象（如上面的BankObject）重写<code>+automaticallyNotifiesObserversForKey:</code>方法（默认返回YES，即默认所有键值都会通知变更），定义需要手动控制的属性。记得在不需要修改的key上返回super方法。</li>
<li>在需要发送通知的地方调用：
<ul>
<li>一对一：<code>willChangeValueForKey:</code>、<code>didChangeValueForKey:</code></li>
<li>有序一对多：<code>willChange:valuesAtIndexes:forKey:</code>、<code>didChange:valuesAtIndexes:forKey:</code></li>
<li>无序一对多：<code>willChangeValueForKey:withSetMutation:usingObjects:</code>、<code>didChangeValueForKey:withSetMutation:usingObjects:</code></li>
</ul></li>
</ol>
<p>如果没有重写相关通知方法，则只会在initial选项生效。</p>
<p>手动处理变更通知的应用场景：</p>
<ul>
<li>筛选通知，如去重、控制通知发送时机。</li>
<li>多个键的通知一起发送。</li>
</ul>
<h3 id="注册依赖键">注册依赖键</h3>
<p>需要在被观察对象（如上面的BankObject）重写<code>+keyPathsForValuesAffectingValueForKey:</code>方法或<code>+keyPathsForValuesAffecting&lt;Key&gt;</code>方法，返回给定key依赖的key path集合。记得在不需要修改的key上返回super方法。</p>
<p>注意：<code>+keyPathsForValuesAffecting&lt;Key&gt;</code>方法在Swift中要声明为<code>@objc</code>。否则不会识别。这种方法还可以用于在分类添加依赖键。</p>
<h3 id="注册集合监听">注册集合监听</h3>
<p>对不可变集合采用一对一的对象赋值方式更新，对于可变集合，访问集合时，不能直接访问对应属性，需要使用<code>mutableArrayValueForKey:</code>类似的方法取出集合，对取出的集合进行增删改都会出发变更通知。</p>
<p>集合注册依赖键比较麻烦，需要使用KVC的方式。</p>
<h3 id="新特性与坑">新特性与坑</h3>
<p>Swift重新封装了注册和移除监听的方式：</p>
<figure class="highlight swift"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 注册与取消</span></span><br><span class="line"><span class="keyword">func</span> <span class="title function_">observe</span>&lt;<span class="type">Value</span>&gt;(<span class="keyword">_</span> <span class="params">keyPath</span>: <span class="type">KeyPath</span>&lt;_KeyValueCodingAndObserving, <span class="type">Value</span>&gt;, <span class="params">options</span>: <span class="type">NSKeyValueObservingOptions</span> <span class="operator">=</span> [], <span class="params">changeHandler</span>: <span class="keyword">@escaping</span> (_KeyValueCodingAndObserving, <span class="type">NSKeyValueObservedChange</span>&lt;<span class="type">Value</span>&gt;) -&gt; <span class="type">Void</span>) -&gt; <span class="type">NSKeyValueObservation</span></span><br><span class="line"><span class="keyword">func</span> <span class="title function_">invalidate</span>()</span><br><span class="line"></span><br><span class="line"><span class="comment">// 使用</span></span><br><span class="line">weather.observe(\.text, options: [.initial, .new, .old, .prior], changeHandler: printSwiftKVOReponse)</span><br></pre></td></tr></table></figure>
<p>iOS 11以下使用时，需要主动置空NSKeyValueObservation或调用<code>invalidate</code>方法。</p>
<p>Swift的KVO方式弱化了observer的存在，而是使用block接收变更通知。并返回一个token，用token进行取消注册。进而替代原有的注册与取消监听的方式。而其他方法observing对象的方法基本不变。</p>
<p>另外<code>@objc dynamic</code>只支持与OC共用的类型，像Swift的枚举、结构体、元组都不支持。</p>
<p>监听OC的枚举类型还会出问题，导致change的所有值都为空，而使用传统的注册监听方式是正常的。</p>
<p>即使测试中监听可选类型的属性也会导致上述问题。</p>
<h2 id="kvo实现原理">KVO实现原理</h2>
<p>使用runtime创建被观察对象的子类，重写sttter，附加KVO通知逻辑，然后把<code>isa</code>指针指向创建的子类并重写相关方法，实现对原本对象的替换。</p>
<p>在运行时根据原类创建一个中间类，这个中间类是原类的子类，并动态修改当前对象的isa指向中间类，并将class方法重写，返回原类的Class。当修改实例对象属性时，会调用Foundation的<code>_NSSetXXXValueAndNotify</code>函数，函数先调用<code>willChangeValueForKey:</code>，然后调用父类原来的setter方法修改值，最后是<code>didChangeValueForKey:</code>，这些方法触发Observer的监听方法。</p>
<h3 id="触发机制">触发机制</h3>
<p>所以，基于原理得知，所有调用属性setter的都会触发KVO通知，所以KVC也会触发KVO，但直接修改成员变量则不会。</p>
<p>注意：使用KVC能对没有setter的属性，甚至成员变量修改值，这都会触发相同keyPath的KVO通知。</p>
<h2 id="参考">参考</h2>
<ul>
<li>在Swift中使用KVO：<a href="../Swift/Language%20Interoperability/Cocoa%20Design%20Patterns/Using%20Key-Value%20Observing%20in%20Swift.md">Using Key-Value Observing in Swift</a></li>
<li><a target="_blank" rel="noopener" href="https://ityongzhen.github.io/%E5%85%B3%E4%BA%8EKVO%E7%9C%8B%E8%BF%99%E7%AF%87%E5%B0%B1%E5%A4%9F%E4%BA%86.html/">关于KVO看这篇就够了 | 殷永振</a></li>
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